1. Field of the Invention
The present invention relates to an exposure device for the reprographic industry, as is used for the exposure of a photosensitive material through a master. The photosensitive material is usually a film, an offset printing plate or a photopolymer printing plate, and the master is usually a film. The photosensitive material and the master together represent the copy partners.
2. Discussion of the Related Art
In the exposure of such photosensitive materials through a master, the quality of the resulting copy decisively depends on the close, completely flush fit of the copy partners against each other. In order to assure this, special measures are typically taken to reliably press the photosensitive material over its entire surface against the master.
Most frequently, a flexible mat, in particular a rubber mat, is used for this purpose, which after the insertion therebetween of the copy partners, can be pressed together with a glass plate through which the exposure is effected. During the pressing together of the rubber mat and the glass plate, sealing contact is established with a sealing bead bordering the mat. In the mat, at least one orifice connectable with a source of negative pressure is provided, and following the generation of negative pressure in the space between the glass plate, the mat and the copy partner (master and photosensitive material) inserted inbetween are suctioned against the glass plate and/or pressed against the glass plate by the higher atmospheric pressure acting on the other surface of the mat.
A constantly recurring problem is the prevention of so-called hollow copies. These are copies with blurred areas resulting from the fact that, in spite of the efforts to establish intimate contact over the entire surface between the copy partners, there still are present locations in which a separation remains between the master and the photosensitive material. In these areas, due to the scattered light, fine lines and half-tone dots as well as with the trueness of the copy are lost.
Numerous attempts have been made to remedy this defect.
Among others, the suctioning is repeated, whereby initially a prevacuum of approximately 40% of the full vacuum (the vacuum obtained with the given source of negative pressure) is applied for approximately 2 minutes and then released, with the idea that the copy partners will now again align and flatten themselves against each other, whereupon the final, full vacuum is applied and a waiting period of a few minutes is observed prior to the exposure. In spite of the considerable time expenditure inherent in this process, the results are not always satisfactory.
It frequently occurs that the rubber mat is already suctioned tightly at the onset of the introduction of the negative pressure against the glass plate in its border areas and then potentially present bulges can no longer be eliminated. If the border areas of the rubber mat adhere tightly against the glass plate, islands of air can no longer be eliminated, even by increasing the negative pressure.
Attempts have been made to establish tight contact of the rubber mat with the glass plate by mechanical action against the rear side of the mat. Thus, for example, according to one proposal in an exposure device having a stationary rubber mat which is positioned on the bottom and upon which the copy partners are placed whereupon the pivoting glass plate is lowered, the rubber mat is supported by sheet metal which is convexly bowed in the upward direction. This functions so that, during the lowering of the glass plate, initially the center of the mat and of the copy partners is pressed against the glass plate, and the pressure zone expands outwardly toward the edges. The idea here is that, in this manner, no air pockets can remain trapped between the master and the photosensitive material, but instead they are pressed out in the outward direction. However, this measure again does not provide the desired success.
Another proposal made in DE-OS 27 19 716 resides in additionally introducing a transparent bag to be filled with a pressure medium between the glass plate and the copy master, which upon pressurization is to assure full contact of the copy partners. This proposal has yet to prove itself, but in any case, it involves a not insubstantial complication of the construction and operation of the copying frame.
Finally, it is known to bring the copy partners into full contact with each other by means of electrostatic forces. In commercial literature of the Simco Company of Landsdale, Pennsylvania, U.S.A., an electrostatic copying frame is described, in which the copy partners are merely placed onto an electrostatically charged surface and smoothed manually by means of a roller rotated with a handle. There is no rubber mat or glass plate, and no negative pressure is used. This entirely different process again does not yield the results desired, and is not entirely without danger for the operator.